1. Field of the Invention
The invention relates to a microelectromechanical device array and a method for driving the microelectromechanical device array at a high speed.
2. Background Art
JP-A-10-48543 (the term “JP-A” as used herein means an “unexamined published Japanese patent application) discloses a conventional method for driving a microelectromechanical device array, such as a DMD (Digital Micro-mirror Device). This conventional driving method will be described with reference to FIGS. 3 to 5.
FIG. 3 is a schematic drawing that illustrating two of an array of microelectromechanical devices that constitute a microelectromechanical device array. A semiconductor substrate 1 contains a drive circuit, not shown, therein, and has movable mirrors 2 and 3 disposed on the surface thereof.
Each of the movable mirrors 2 and 3 is supported in a space by a hinge 6 extended between supporting rods 4 and 5 erected on the surface of the semiconductor substrate 1, and can swing right and left upon the hinge 6. Movable electrode films 7 and 8 are formed integrally with the hinge 6 at the right and the left of the hinge 6 placed therebetween, respectively. Fixed electrode films 9 and 10 are formed on the surface of the semiconductor substrate 1 at positions facing the movable electrode films 7 and 8, respectively.
A bias voltage Vb of 24V (Vb=24V) is applied to the hinge 6 (i.e., the electrode films 7 and 8) of the movable mirror 2 as a control voltage. An address voltage Va of 5V (Va=5) is applied to the fixed electrode film 9 as a displacement signal, and an address voltage Va of 0V (Va=0) is applied to the fixed electrode film 10 as a displacement signal. As a result, a voltage difference DV of 19V (DV=19) is caused between the electrode films 7 and 9, and a voltage difference DV of 24V (DV=24V) is caused between the electrode films 8 and 10. Therefore, the movable mirror 2 is tilted in a direction in which the electrode films 8 and 10 come into contact with each other by a difference between an electrostatic force generated between the electrode films 7 and 9 and an electrostatic force generated between the electrode films 8 and 10. FIG. 3 illustrates a state in which the movable mirror 2 is tilted by −10°.
Likewise, a bias voltage Vb of 24V (Vb=24V) is applied to the hinge 6 (i.e., the electrode films 7 and 8) of the movable mirror 3. An address voltage Va of 0V (Va=0) is applied to the fixed electrode film 9, and an address voltage Va of 5V (Va=5) is applied to the fixed electrode film 10. As a result, a voltage difference DV of 24V (DV=24) is caused between the electrode films 7 and 9, and a voltage difference DV of 19V (DV=19V) is caused between the electrode films 8 and 10. Therefore, the movable mirror 3 is tilted in a direction in which the electrode films 7 and 9 come into contact with each other by a difference between an electrostatic force generated between the electrode films 7 and 9 and an electrostatic force generated between the electrode films 8 and 10. FIG. 3 illustrates a state in which the movable mirror 3 is tilted by +10°.
When an incident light is projected onto the movable mirrors 2 and 3, light that has impinged thereon is reflected therefrom in various directions depending on the tilt of the movable mirrors 2 and 3. Therefore, the direction of the reflected light can be on-off-controlled by controlling the tilt of the movable mirrors 2 and 3.
However, it is difficult to tilt the movable mirror, which has been already tilted, in an opposite direction, and hence a conventional method has been employed in which the movable mirror is controllably driven while performing complex voltage control. This will be described with reference to FIG. 4 and FIG. 5.
The tilted movable mirror 2 is illustrated at the uppermost part of FIG. 4. If the movable mirror that has been tilted toward left side is brought into a next state, two cases can be mentioned as the “next state”. The two cases are a case in which the movable mirror is tilted toward the opposite side (right side) and a case in which the movable mirror is tilted toward the same side (left side), i.e., the tilted state is kept unchanged. If this microelectromechanical device array is used in an image forming apparatus, the state to be reached depends on data about an image to be formed.
The left in each frame illustrated at the lower part of FIG. 4 illustrates a case in which the movable mirror 2 is displaced toward the opposite side (i.e., crossover transition), whereas the right therein illustrates a case in which the tilted state of the movable mirror 2 is maintained (i.e., stay transition). Address voltages Va applied to the fixed electrode films 9 and 10 of each of the movable mirrors 2 and 3 are controlled individually in the movable mirrors 2 and 3, whereas a common bias voltage Vb is applied to all of the movable mirrors.
When the tilted state of the movable mirror is changed to the next state, the bias voltage Vb is changed as illustrated in FIG. 5. Let the period from the start of the change of the movable mirror to the end thereof be divided into zone A, zone B, zone C, zone D, and zone E. First, the bias voltage Vb is set at 24V (Vb=24V) in zone A, and the bias voltage Vb is set at 26V (Vb=26V) in zone B. Further, the bias voltage Vb is set at 7.5V (Vb=7.5V) in zone C following zone B, and the bias voltage Vb is returned to 24V (Vb=24V) in zone D. The bias voltage Vb is kept at 24V (Vb=24V) in zone E.
In zone A, the address voltage Va (0V or 5V) is rewritten. When the movable mirror is changed to the next state, the movable electrode films 7 and 8 moved together with the movable mirror are brought close to the fixed electrode film 9. When the movable mirror is intended to be tilted, the voltage Va to be applied to the fixed electrode film 9 is set at 0V. When the movable mirror is intended to be tilted while bringing the movable mirror close to the fixed electrode film 10, the voltage Va to be applied to the fixed electrode film 10 is set at 0V, and the voltage Va to be applied to the fixed electrode film disposed on the opposite side is set at 5V.
When the applied voltage Va is controlled in this way, the bias voltage Vb comes to −26V (Vb=−26V) in zone B as illustrated at the left (i.e., crossover side) of FIG. 4. Accordingly, a voltage difference DV of 33.5V (DV=33.5V) is generated between the electrode films 8 and 10, and a voltage difference DV of 26V (DV=26V) is generated between the electrode films 7 and 9. As a result, the movable mirror 2 receives an electrostatic force by which the movable mirror 2 is tilted toward left, and the movable electrode film 8 is pressed against the fixed electrode film 10, and is elastically deformed.
When the bias voltage Vb comes to 7.5V (Vb=7.5V) in zone C following zone B, voltage Va to be applied to the address electrode film (i.e., fixed electrode film) 10 is set at 7.5V (Va=7.5V) . As a result, a voltage difference DV of 0V (DV=0) is generated between the electrode films 8 and 10, and a voltage difference DV of 7.5V (DV=7.5V) is generated between the electrode films 7 and 9. Accordingly, an electrostatic force is generated between the electrode films 7 and 9, and a repulsive force generated by the elastic deformation of the movable electrode film 8 in zone B is added to the electrostatic force, so that the movable electrode film 8 is separated from the fixed electrode film 10, and the movable mirror 2 starts being rotated clockwise.
When the bias voltage Vb comes to 24V (Vb=24V) in zone D following zone C, a voltage-difference DV of 16.5V (DV=16.5V) is generated between the electrode films 8 and 10, and a voltage difference DV of 24V (DV=24V) is generated between the electrode films 7 and 9. As a result, the electrostatic force acting between the electrode films 7 and 9 is further increased, and the movable mirror 2 is further rotated clockwise.
In zone E that is the last zone, the movable electrode film 7 of the movable mirror 2 strikes the address electrode film 9. At this time, voltage Va to be applied to the address electrode film 10 is set at 5V (Va=5V) . Because of this collision, the movable mirror 2 slightly vibrates as illustrated in FIG. 5, and reaches a stable state, thus ending its tilt-action performed toward the opposite side.
To bring the movable mirror 2 into the state illustrated at the right (i.e., stay side) of FIG. 4, voltage Va to be applied to the address electrode film (fixed electrode film) 10 is set at 0V (Va=0) as illustrated at the upper part of the right in the frame of FIG. 4 (zone A) . When the bias voltage Vb reaches −26V (Vb=−26V) in zone B following zone A, voltage Va to be applied to the address electrode film (fixed electrode film) 9 disposed on the opposite side is set at 7.5V (Va=7.5V), and the bias voltage Vb is set at 7.5V (Vb=7.5V) in zone C.
In this case, when the movable electrode film 8 is temporarily separated from the fixed electrode film 10, and the bias voltage Vb comes to 24V (Vb=24V) in zone D as illustrated by the dotted round mark H of FIG. 5, the movable electrode film 8 again comes into contact with the fixed electrode film 10. Thereafter, voltage Va to be applied to the fixed electrode film 9 in zone E is set at 5V (Va=5V), and the tilted state of the movable mirror 2 is kept to be tilted toward left. In the above description, the term “contact” is used for convenience of explanation, however, a gap is formed between the movable electrode film and the fixed electrode film, and hence an electric short circuit is never caused between the electrode films. The same applies to a description given below.
According to aforesaid conventional method for driving a microelectromechanical device array, address rewriting (i.e., application of voltage Va) to change the state to the next state is performed after waiting for the end of zone E, i.e., after waiting for the end of the vibration of the movable mirror. The reason is as follows. If address rewriting is performed while the movable mirror is vibrating, e.g., if address rewriting is performed to tilt the movable mirror toward right while the left-tilted movable mirror is vibrating, a vibrating force is added to the electrostatic force added to the movable mirror, so that the movable mirror is immediately tilted toward right in most cases. As a result, light reflection cannot be performed in the left-tilted state, and this will cause a malfunction.
Therefore, according to the conventional method, next-address rewriting (zone A) is performed after waiting for the end of zone E (in the example illustrated in FIG. 5, after a lapse of 22 μs), and hence the microelectromechanical device array has difficulty in operating at high speed.
If address rewriting (zone A) can be performed without malfunction immediately after the start of zone E, the process can proceed to zone B and zone C anytime after the end of the vibration in zone E, and the microelectromechanical device array can operate at high speed. However, there is no conventional technique for ensuring the address rewriting without malfunction.